[RISCV] Add callback plumbing for getArithmeticInstrCost [nfc]

Most of the code for this was taken from https://reviews.llvm.org/D133552, with one bug fix by me.  I'm landing the plumbing so that we can focus on the cost model pieces in the review.

Author: preames

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -993,6 +993,40 @@ InstructionCost RISCVTTIImpl::getVectorInstrCost(unsigned Opcode, Type *Val,
   return BaseCost + SlideCost;
 }
 
+InstructionCost RISCVTTIImpl::getArithmeticInstrCost(
+    unsigned Opcode, Type *Ty, TTI::TargetCostKind CostKind,
+    TTI::OperandValueInfo Op1Info, TTI::OperandValueInfo Op2Info,
+    ArrayRef<const Value *> Args, const Instruction *CxtI) {
+
+  // TODO: Handle more cost kinds.
+  if (CostKind != TTI::TCK_RecipThroughput)
+    return BaseT::getArithmeticInstrCost(Opcode, Ty, CostKind, Op1Info, Op2Info,
+                                         Args, CxtI);
+
+  if (isa<FixedVectorType>(Ty) && !ST->useRVVForFixedLengthVectors())
+    return BaseT::getArithmeticInstrCost(Opcode, Ty, CostKind, Op1Info, Op2Info,
+                                         Args, CxtI);
+
+  // Skip if scalar size of Ty is bigger than ELEN.
+  if (isa<VectorType>(Ty) && Ty->getScalarSizeInBits() > ST->getELEN())
+    return BaseT::getArithmeticInstrCost(Opcode, Ty, CostKind, Op1Info, Op2Info,
+                                         Args, CxtI);
+
+  // Legalize the type.
+  std::pair<InstructionCost, MVT> LT = getTypeLegalizationCost(Ty);
+
+  // TODO: Handle scalar type.
+  if (!LT.second.isVector())
+    return BaseT::getArithmeticInstrCost(Opcode, Ty, CostKind, Op1Info, Op2Info,
+                                         Args, CxtI);
+
+  // TODO: Put a real cost model here, the plumbing change was landed without
+  // one to simplify review and fault isolation.
+
+  return BaseT::getArithmeticInstrCost(Opcode, Ty, CostKind, Op1Info, Op2Info,
+                                       Args, CxtI);
+}
+
 void RISCVTTIImpl::getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
                                            TTI::UnrollingPreferences &UP,
                                            OptimizationRemarkEmitter *ORE) {

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h

@@ -155,6 +155,13 @@ class RISCVTTIImpl : public BasicTTIImplBase<RISCVTTIImpl> {
   InstructionCost getVectorInstrCost(unsigned Opcode, Type *Val,
                                      unsigned Index);
 
+  InstructionCost getArithmeticInstrCost(
+      unsigned Opcode, Type *Ty, TTI::TargetCostKind CostKind,
+      TTI::OperandValueInfo Op1Info = {TTI::OK_AnyValue, TTI::OP_None},
+      TTI::OperandValueInfo Op2Info = {TTI::OK_AnyValue, TTI::OP_None},
+      ArrayRef<const Value *> Args = ArrayRef<const Value *>(),
+      const Instruction *CxtI = nullptr);
+
   bool isElementTypeLegalForScalableVector(Type *Ty) const {
     return TLI->isLegalElementTypeForRVV(Ty);
   }